1. Field of the Invention
This invention pertains to one-shot or one-component epoxy resin systems and latent catalysts therefor. More particularly, it addresses modified imidazoles useful as latent catalysts in such systems.
2. Background of the Prior Art
Increasing attention, particularly in the composites field, has been concentrated on "one-shot" or "one-component" epoxy resin systems, as having excellent mechanical properties for a variety of applications, including prepregs, laminates, bonding adhesives, coatings, finishes, tooling resins, pultrusion, etc., without requiring sophistication on the part of the user, or sophisticated apparatus.
Unfortunately, these systems, in which a latent catalyst is dispersed in the unpolymerized or uncured resin, inactive at room temperatures or lower, have been plagued by several problems. Chief amongst these problems is the limited shelf-life of these single package systems. They tend to cure, or polymerize, rapidly and prematurely, in a matter of days, up to a week or two. Of course, this puts extreme demands on inventory, and price, and similarly, makes the material difficult to work with on a large project, which takes a substantial amount of time to bring to completion.
Another problem encountered is the heat levels, or prolonged exposure to high heat levels, necessary to cure these systems.
As an example, imidazole, and imidazole derivatives have been used as latent catalysts giving polymeric materials that cure quickly, and have excellent mechanical characteristics. However, these have a very short shelf life, 1-2 days at most at ambient conditions. Any processing delays encountered may result in a loss of half the resin or more, as a result of its short shelf life.
There have been prior art attempts to overcome these problems. Thus, U.S. Pat. No. 4,335,228, describes isocyanate blocked imidazoles/imidazolines for use as latent catalysts in epoxy resin systems. However, such catalyst must be used with epoxy resins that have a melting temperature of 60.degree. C. or above. They also require cure temperatures of 250-400.degree. F.
European patent application No. 0024119 describes a one-package system using a succinic acid salt of a phenylisocyanate-modified imidazole as a latent catalyst. However, when combined with an epoxy resin prepared from polyglycidyl ether, the resulting product was found to have only 2-3 days shelf life at room temperature.
Of course, other catalysts, such as dicyandiamide, are known, having excellent shelf-life. However, the curing temperatures of these, 350.degree. F. and above, are too high for many preferred applications. Of course, curing may be accelerated by addition of various salts of amines, and ureas. However, these additives dramatically reduce the shelf-life of the resin system, and adversely affect the properties of the resulting cured product.
Accordingly, it remains a goal of the art to provide a one-package epoxy resin system, comprising a latent catalyst, which results in good mechanical properties in the cured product, while exhibiting an extended shelf-life.